In an electrostatographic process a latent electrostatic image is formed on a photoconductor. That image can be developed by means of a rotating magnetic brush that consists of small magnetic carrier particles which, under the influence of magnets in the core of the brush, form fur-like hairs extending from the core. The magnetic brush triboelectrically charges toner particles and carries those charged toner particles to the oppositely charged electrostatic latent image on the photoconductor, thereby developing the image. The developed image is then transferred to a receiver such as paper.
In copending U.S. patent application Ser. No. 62,023, filed June 15, 1987 by E. T. Miskinis et al., titled "Electrographic Magnetic Carrier Particles," now U.S. Pat. No. 4,764,445, there is described hard ferrite magnetic carrier particles, for use in two component developers, which contain from about 1 to about 5% by weight of lanthanum. The presence of lanthanum in the ferrite increases the conductivity of the ferrite and improves its development efficiency.
Japanese Patent Laid-Open No. 124564/1987 (Application No. 263684/1985) titled "Carrier for Developing Electrostatic Charge Images," describes two-phase ferrite carrier particles where one phase forms the core of the particle and a second phase forms the shell of the particle. The core has a hexagonal structure and a mixture of hexagonal and spinel structures and consists of 5 to 30% BaO, 5 to 30% ZnO, and 5 to 90% Fe.sub.2 O.sub.3. The shell has a spinel structure and is formed from a ferrite slurry consisting (in mole percentage) of 5 to 20% NiO, 5 to 35% ZnO, and 40 to 70% Fe.sub.2 O.sub.3, or a ferrite slurry in which a part of the aforementioned components is substituted by one or two or more sorts of metal of univalency or greater.